1. Field of the Invention
The present invention concerns a method to acquire spin echo-based magnetic resonance (MR) signals of an examination subject with a multi-spin echo sequence in which multiple refocusing pulses are radiated after each RF excitation pulse, and an MR system for implementing such a method.
2. Description of the Prior Art
Magnetic resonance diffusion-weighted imaging of the brain with multiple echoes per excitation is very sensitive to spatially variant, non-linear phase errors. These errors are caused, among other things, by the pulsing cerebrospinal fluid (CSF) of the brain that occurs upon switching of the diffusion-weighted gradients (Miller K L, Pauly J P. Nonlinear phase correction for navigated diffusion imaging. Magn. Reson. Med. 2003; 50:343-353). For example, phase images of a single-shot, diffusion-weighted echoplanar imaging sequence are shown in FIG. 1 herein, wherein it can be seen that the phase curve across the image changes from exposure-to-exposure. This leads to severe image artifacts in sequences with multiple excitations and/or multiple refocusing pulses for each image in the event that no correction is made.
Multiple imaging sequences with a repeat excitation were developed in which a correction of these non-linear phase errors is implemented afterwards (Pipe J G, Farthing V G, Forbes K P. Multishot diffusion-weighted FSE using PROPELLER MRI. Magn. Reson. Med. 2002; 47:42-52; Liu C, Bammer R, Kim D, Moseley M E. Self-navigated interleaved spiral (SNAILS): application to high-resolution diffusion tensor imaging. Magn. Reson. Med. 2004; 52:1388-1396; Wang F-N, Huang T-Y, Lin F-H, Chuang T-C, Chen N-K, Chung H-W, Chen C-Y, Kwong K K. PROPELLER EPI: an MRI technique suitable for diffusion tensor imaging at high field strength with reduced geometric distortions. Magn. Reson. Med. 2005; 54:1232-1240; Atkinson D, Counsell S, Hajnal J V, Batchelor P G, Hill D L, Larkman D J. Nonlinear phase correction of navigated multi-coil diffusion images. Magn. Reson. Med. 2006; 56:1135-1139; Porter D A, Heidemann R. High Resolution Diffusion Weighted Imaging Using Readout-Segmented Echo-Planar Imaging, Parallel Imaging and a Two-Dimensional Navigator-Based Re-Acquisition. Magn. Reson. Med. 2009; 62:468-475). These sequences, however, impose limitations in the signal readout of the detected MR signal.
The difficulties in fast spin echo sequences (turbo spin echo sequence, TSE, or fast spin echo (FSE), namely in the readout of the signals during a train of 180° refocusing pulses, are particularly significant. This type of imaging sequence is attractive in the clinical routine since no susceptibility artifacts (which increasingly arise in other methods) occur. In this case an additional difficulty is that phase errors occur due to the random phase difference between the 90° excitation pulse and the 180° excitation pulse. This means that the Carr-Purcell-Meiboom-Gill (CMPG) condition, which is necessary in order to stabilize the amplitude and phase of the signal train of the spin echo, is not satisfied. A few techniques are known that remedy this problem of the fast spin echo sequences by cyclically switching the radiation direction of the refocusing pulses (Pipe J G, Farthing V G, Forbes K P. Multishot diffusion-weighted FSE using PROPELLER MRI. Magn. Reson. Med. 2002; 47:42-52; Bastin M E, Le Roux P. Application of non-CPMG fast-spin-echo sequences to MR diffusion imaging. Proc. Intl. Soc. Magn. Reson. Med. 2001; 9:1549), or by using stimulated echoes in order to avoid the phase variation (Alsop D C. Phase insensitive preparation of single-shot RARE: application to diffusion imaging in humans. Magn. Reson. Med. 1997; 38:527-533; SPLICE: sub-second diffusion-sensitive MR imaging using a modified fast spin-echo acquisition. Schick F. Magn. Reson. Med. 1997; 38:638-644). However, the cyclical change of the radiation axis does not stabilize the echo train in all cases, and a signal reduction results given the use of the stimulated echo. In Norris D G, Driesel W. Online motion correction for diffusion-weighted imaging using navigator echoes: Application to RARE imaging without sensitivity loss. Magn. Reson. Med. 2001; 45:729-733, a prospective (or predictive) correction of linear phase errors is described in which data of a navigator signal are used to switch a gradient before the data acquisition. This method was used in the original with a 1D navigator technique in order to correct a linear phase error along one spatial direction, In Weih K S, Driesel W, von Mengershausen M, Norris D G. Online motion correction for diffusion-weighted segmented EPI and FLASH imaging. MAGMA 2004; 16:277-283, orthogonal navigator signals were used in order to correct k-space shifts in two dimensions.
A number of RF transmission channels were used in order to control the spatial distribution of a B1 amplitude of an RF excitation pulse and the phase of the excitation pulse in order to achieve an improved homogeneity in the image (Zhu Y. Parallel excitation with an array of transmit coils. Magn. Reson. Med. 2004; 51:775-784; Zhu Y, Giaquinto R. Improving flip angle uniformity with parallel excitation. Proc. Int. Soc. Magn. Reson. Med. 2005; 13:2752; Collins C M, Liu W, Swift B J, Smith M B. Combination of optimized transmit arrays and some receive array reconstruction methods can yield homogeneous images at very high frequencies. Magn. Reson. Med. 2005; 54:1327-1332).